Role of nuclear data in the practical application of nondestructive nuclear assay methods Page: 3 of 21
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niques under investigation as well as for design optimization of systems al-
ready proven to be worthy of development to the prototype stage. The third
category utilizes nuclear data as a reservoir of basic information to aid in
the search for new methods and signatures.
FUEL PIN ASSAY
A number of fuel pin assay systems either complete or under development
serve as an illustration of some of these uses of nuclear data. These sys-
tems are for the measurement of:
1. Uranium-235 content of LWR fuel.
2. Uranium-235 content of LWR fuel and the detection of out of
specification fuel pellets.
3. Plutonium content and isotopic composition of fast breeder
reactor (FBR) fuel.
4. Plutonium recycle LWR fuel.
5. Fissile content of irradiated fuel.
6. Uranium-235 content of LWR fuel which also contains burnable
Three properties of the system for measuring LWR pins are most
1. High throughput rate to take care of a modern plant capacity of
several hundred pins per shift.
2. Sensitivity to all of the fissile material within the pins.
These three criteria lead to the choice of thermal neutron interrogation;
the thermal flux supplied by a moderated Cs2Cf source and the detection of
prompt fission neutrons from the thermal fission of 2 sU furnishing the pri-
mary signature and differentiation from 2l*U, the major constituent of LWR
The moderator for the neutron source must provide sufficient thermal
L.x at the fuel channel while providing maximum discrimination between 23SU
and 3eeU as well as minimizing the source strength and biological shielding
required. The configuration of source, moderator materials and detectors
was optimized by modeling the system on the computer using only enough actual
experimentation to assure confidence in the computer calculations (1). The
method of optimizing a design by computer calculations illustrates one of the
more important uses of nuclear data as input to computer codes which provide
the calculational tool necessary to design practical nondestructive assay
The assay of a fuel pin requires a mathematical calibration function
which describes as accurately as possible the response for various enrich-
ments and yet is simple enough to permit on-line data reduction. Analysis [2J
indicates that the response of a fuel rod having an enrichment less than 4t
29fU is adequately described by a function of the form
R - A(l - ebU) .
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Thorpe, M.M. Role of nuclear data in the practical application of nondestructive nuclear assay methods, article, October 31, 1973; New Mexico. (digital.library.unt.edu/ark:/67531/metadc1030175/m1/3/: accessed May 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.